Ctenophora

Introduction

Ctenophora (tĭnŏfˈərə) [key], a small phylum of exclusively marine, invertebrate animals, commonly known as sea walnuts or comb jellies. Because they are so delicate that specimens are difficult to collect, little was known about them until the advent of blue-water scuba and submersible collecting. Ctenophores are characterized by eight rows consisting of ciliated plates called ctenes (combs), which are radially arranged on the spherical body surface. The animals swim weakly, powered by those structures. The two hemispheres of the ctenophore body are marked by a mouth, or oral pole, on the underside, and an opposite aboral pole, on which is located the statocyst, a unique sense organ controlling equilibrium. Most ctenophores resemble biradially symmetrical (see symmetry, biological) jellyfish (phylum Cnidaria) but lack the cnidarian whorl of tentacles around the mouth. They lack the specialized stinging cells (nematocysts) found in coelenterates, but one species (Haeckelia rubra) incorporates those of its jellyfish prey for its own defense. Ctenophores, which are all carnivorous, have specialized adhesive cells called colloblasts, used to capture planktonic animals on which the ctenophores feed. Approximately 50 species are known, but many become locally abundant and are ecologically significant. They vary from less than 1/4 in. (0.6 cm) to over 1 ft (30.5 cm) long. Most are transparent, but pale pinks, reds, violets, and oranges are also known in some species. Most ctenophores are also bioluminescent, the production of light originating in the walls of the eight canals. Most ctenostomes are hermaphrodites, developing through a cydippid larval stage to adults. They can also regenerate lost parts.